compositor-worker: Initialize CW machinery plumbing to compositor and fire CW rAF callbacks.

cc/trees/threaded_channel.cc

Index: cc/trees/threaded_channel.cc
diff --git a/cc/trees/threaded_channel.cc b/cc/trees/threaded_channel.cc
index eacfabbdd821e5a18c08e8528604bcf1497cc6c7..a4da549722cd919f89cd18d1a66f4dd3d3cdce42 100644
--- a/cc/trees/threaded_channel.cc
+++ b/cc/trees/threaded_channel.cc
@@ -9,6 +9,7 @@
 #include "base/single_thread_task_runner.h"
 #include "base/trace_event/trace_event.h"
 #include "cc/animation/animation_events.h"
+#include "cc/animation/layer_tree_mutator.h"
 #include "cc/trees/layer_tree_host.h"
 
 namespace cc {
@@ -54,6 +55,14 @@ void ThreadedChannel::InitializeOutputSurfaceOnImpl(
                             proxy_impl_weak_ptr_, output_surface));
 }
 
+void ThreadedChannel::InitializeMutatorOnImpl(
+    std::unique_ptr<LayerTreeMutator> mutator) {
+  ImplThreadTaskRunner()->PostTask(
+      FROM_HERE,
+      base::Bind(&ProxyImpl::InitializeMutatorOnImpl, proxy_impl_weak_ptr_,
+                 base::Passed(std::move(mutator))));
+}
+
 void ThreadedChannel::MainThreadHasStoppedFlingingOnImpl() {
   DCHECK(IsMainThread());
   ImplThreadTaskRunner()->PostTask(